Preliminary studies done in our laboratory have indicated that O-methylated metabolites of catechols are demethylated in erythrocytes. This activity is different from that of demethylases which are localized in liver microsomes, in that the addition of reduced nicotinamide adenine dinucleotide phosphate is inhibitory to the reaction and formaldehyde is not a product. We propose to isolate the enzyme responsible for demethylation in red blood cells, using a variety of techniques including gel filtration, ion exchange chromatography, column isoelectric focussing procedure, affinity chromatography and gel electrophoresis. Following the acquisition of a demonstrably homogeneous preparation of the enzyme, as analyzed by electrophoretic techniques, the gross physical and chemical properties of the protein will be determined. The substrate specificity and kinetic properties of the purified enzyme will also be studied.